1. Field of the Invention
The present invention relates to an antireflection structure formation method and an antireflection structure.
2. Background Art
According as ultrafine micro-fabrication technologies used in LSI production processes and micro-machining processes have been developed recently, it has become possible to produce sub-wavelength gratings, which have periods shorter than the wavelength of light and which can be processed in nanometer scale. For example, as one of the sub-wavelength gratings, there is proposed a non-reflective periodic structure (see, Hiroshi Toyota, Kogaku (Japanese Journal of Optics [in Japanese]), Optical society of Japan, vol. 32 (2003), pp. 489). The non-reflective periodic structure has a surface provided with numerous small projections by which Fresnel reflection on the light-incident surface is reduced to ensure aimed antireflection properties. The Fresnel reflection is generally determined by inherent refractive index of substance. However, in the non-reflective periodic structure, the refractive index is artificially set up with the surface structure formed more finely than the wavelength of incident light and is also made to vary continuously from the light-incident side to the substrate side, and thereby the aimed antireflection properties can be realized. This antireflection structure can reduce reflectance in a wide wavelength range and in a wide incident angle range, and accordingly is effectively used for optical elements or devices, such as lenses and displays, in which light reflection often becomes a large problem.
In order to obtain the above antireflection effect, the intervals among the projections must be several hundred nanometers or less and hence the fabrication in nanometer scale is required. In view of this, it is proposed to fabricate an antireflection structure by electron beam lithography or by laser beam interference exposure method (see, P. Lalanne, et. al., Nanotechnology, 8, 52 (1997)). However, although fine patterns can be formed very precisely, these fabrication technologies are not suitable for industrial applications because they need expensive apparatuses and give low throughput.
It is also proposed to form nano-size projections not by the conventional lithographic technologies but by an etching process in which particles are used as an etching mask (JP-A 2006-512781(KOKAI)). According to this etching process, columnar structures having sizes corresponding to diameters of the particles can be formed on a substrate. However, in the process, the selective etching ratio between the particles and the substrate is too large to form projections having high antireflection functions. It is also proposed to form projections by another etching process in which particles having a high selective etching ratio to the substrate is used as the mask. In the process, first the substrate is processed and then the particles are slimed while the etching gas is successively changed (JP-A 2005-331868 (KOKAI)). However, although projections can be almost obtained, this etching process comprises complicated procedures since the etching gas must be successively changed. Further, bumps are formed in the projections whenever the etching gas is changed, so that the refractive index varies not smoothly and, as a result, that the antireflection functions are impaired. Furthermore, since all the particles serving as the mask are completely etched, the tips of the projections are so sharpened by side-etching that the refractive index changes steeply at the tips to lower the antireflection functions. This unfavorable effect is enhanced if the particles have more uneven sizes, and consequently the projections are liable to have such uneven shapes that the antireflection functions are deteriorated and/or that the process margin is often narrowed to lower the productivity.